This invention relates in general to synthesizers of periodic signals of selectable frequency commonly referred to as "frequency synthesizers" and in particular to frequency synthesizers which utilize direct mix and filter stages in their synthesis process.
Some conventional frequency synthesizers utilize a plurality of serially ganged mix and filter stages, such as the commonly referred to "decade" stages, in their synthesis process. Each stage performs the function of mixing an input periodic signal with one or more mix frequencies generated by a mix frequency generator, and dividing the mixed signal by ten, the input periodic signal being the output signal from a preceding stage or, if the first stage, a reference oscillator signal from an external source, typically 10 Mhz. In a synthesizer using decade stages, each stage adds one decimal digit of resolution. The number of stages depends on the desired resolution of the output signal. In each stage, all of the functions are linear except for the divider, which is a nonlinear device causing transient anomalies at frequency switching. A change in frequency causes a transient condition in which the divider's state is undefined. The non-linearity of the divider causes the phase of the synthesizer output signals to be indeterminable.
For example, when first selecting a signal of frequency X, the phase of the signal is unpredictable, and likewise when reselecting frequency X from one or more intervening frequencies, the phase is not only unpredictable, but it is not necessarily the same phase as when the frequency X was first selected, there being an ambiguity of ten in the phase of the signal per decade stage divider when returning to frequency X.
Heretofore, the prior art has not presented a means nor a method which can keep the phase of a given output frequency constant no matter how many times the frequency is selected and reselected. This invention presents a means and a method adapted to make the phase of all selected frequencies determinate.
Other advantages and attributes of this invention will be readily discernible upon the reading of the text hereinafter.